1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an organic electroluminescent display device and a method of fabricating the same.
2. Discussion of the Related Art
Among flat panel displays, organic electroluminescent displays have properties of high brightness and low driving voltages. In addition, because they are self-luminous, the organic electroluminescent displays have excellent contrast ratios and ultra thin thicknesses. The organic electroluminescent displays have response time of several micro seconds, and there are advantages in displaying moving images. The organic electroluminescent displays have wide viewing angles and are stable under low temperatures. Since the organic electroluminescent displays are driven by low voltage of direct current (DC) 5V to 15V, it is easy to design and manufacture driving circuits. A manufacturing process of an organic electroluminescent display is very simple because only deposition and encapsulation steps are required.
The organic electroluminescent displays are classified into a passive matrix type and an active matrix type. In the passive matrix type, scan lines and signal lines cross each other to form diodes, and the signal lines are sequentially scanned to drive pixels. To obtain required average brightness, instant brightness is needed which is the product of average brightness and the number of lines.
On the other hand, in the active matrix type, a thin film transistor, as a switching element, is formed in each sub-pixel. A first electrode connected to the thin film transistor turns on/off by the sub-pixel, and a second electrode facing the first electrode functions as a common electrode. In addition, a voltage applied to the sub-pixel is stored in a storage capacitor, and the voltage is maintained until the signal of next frame is applied. Accordingly, regardless of the number of the scan lines, the sub-pixels are continuously driven during one frame. Even though low currents are applied, uniform brightness may be obtained. Therefore, recently, the active matrix type organic electroluminescent displays have been widely used because of their low power consumption, high definition and large-sized possibility.
FIG. 1 is an equivalent circuit diagram illustrating a pixel of an active matrix organic electroluminescent display device according to the related art.
In FIG. 1, the pixel of the active matrix organic electroluminescent display device includes a switching thin film transistor STr, a driving thin film transistor DTr, a storage capacitor StgC, and an organic electroluminescent diode E.
More particularly, a gate line GL is formed along a first direction. A data line DL is formed along a second direction crossing the first direction. The gate line GL and the data line DL define a pixel region P. A power line PL for supplying a source voltage is spaced apart from the data line DL.
The switching thin film transistor STr is formed at a crossing portion of the gate line GL and the data line DL. The driving thin film transistor DTr is electrically connected to the switching thin film transistor STr. The organic electroluminescent diode E includes a first electrode connected to a drain electrode of the driving thin film transistor DTr and a second electrode connected to the power line PL. The power line PL supplies the source voltage to the organic electroluminescent diode E. The storage capacitor StgC is formed between a gate electrode and a source electrode of the driving thin film transistor DTr.
A scan signal is applied to the switching thin film transistor STr through the gate line GL, and the switching thin film transistor STr turns on. Then, a data signal from the data line DL is supplied to the gate electrode of the driving thin film transistor DTr, and the driving thin film transistor DTr turns on. Accordingly, the organic electroluminescent emits light. Here, when the driving thin film transistor DTr is in on-state, levels of currents flowing in the organic electroluminescent diode E from the power line PL are determined. The organic electroluminescent diode E produces gray scales according to the levels of the currents. When the switching thin film transistor STr turns off, the storage capacitor StgC maintains a gate voltage of the driving thin film transistor DTr constant. Even though the switching thin film transistor STr is in off-state, the levels of the currents flowing in the organic electroluminescent diode D are constantly maintained until a next frame.
The organic electroluminescent display device is classified into a top emission type and a bottom emission type according to a direction of light emitted from the organic electroluminescent diode. The bottom emission type has a disadvantage of low aperture ratio, and recently the top emission type has been widely used.
FIG. 2 is a schematic plan view of a top emission type organic electroluminescent display device according to the related art, and FIG. 3 is a cross-sectional view taken along the line III-III in FIG. 2.
In FIGS. 2 and 3, the related art top emission type organic electroluminescent display device includes first and second substrates 10 and 70, which are disposed to face each other. Peripheries of the first and second substrates 10 and 70 are sealed by a seal pattern 80. A driving thin film transistor DTr is formed in each pixel region P on the first substrate 10. A passivation layer 40 is formed on the driving thin film transistor DTr and has a drain contact hole 43. A first electrode 47 is formed on the passivation layer 40 and contacts an electrode (not shown) of the driving thin film transistor DTr through the drain contact hole 43.
An organic emission layer 55 is formed on the first electrode 47. The organic emission layer 55 includes red, green and blue luminous patterns 55a, 55b and 55c each corresponding to the pixel region P. A second electrode 58 is formed on the organic emission layer 55 all over the surface of the first substrate 10. The first and second electrodes 47 and 58 provide electrons and holes.
The first substrate 10 and the second substrate 70 are attached by the seal pattern 80, and the second electrode 58 on the first substrate 10 is spaced apart from the second substrate 70.
In the related art top emission type organic electroluminescent display device 1, a buffer pattern 50 is formed in a border area of the pixel region P. The buffer pattern 50 overlaps edges of the first electrode 47. A spacer 51 having a dam shape is formed on the buffer pattern 50 and along a direction of the gate line (not shown), that is, a sequential and alternate arrangement direction of the red, green and blue luminous patterns 55a, 55b and 55c. The spacer 51 supports a shadow mask (not shown), which is used to form the organic emission layer 55.
FIGS. 4A and 4B are plan views of showing a step for forming an organic emission layer using a shadow mask in processes of manufacturing a related art top emission type organic electroluminescent display device. FIG. 5A is a cross-sectional view taken along the line VA-VA of FIG. 4A, and FIG. 5B is a cross-sectional view taken along the line VB-VB of FIG. 4B. FIG. 6 is a cross-sectional view taken along the line IV-IV of FIG. 4B.
An organic emission layer 55 is formed on a first substrate 10 using a shadow mask 90, which includes openings OA corresponding to the pixel regions P and blocking portions BA corresponding to other regions excluding the pixel regions P.
First, the shadow mask 90 is disposed over the substrate 10, on which the organic emission layer 55 will be formed. The shadow mask 90 is aligned such that the opening OA of the shadow mask 90 corresponds to an area for forming the organic emission layer 55 in the pixel region P on the substrate 10. At this time, the shadow mask 90 is located very close to the substrate 10. Since the shadow mask 90 is made of a metallic material, protrusions (not shown) may be formed at a surface of the shadow mask 90 or particles may be attached to the surface of the shadow mask 90. The protrusions or particles may have a size of 1 micrometer to 6 micrometers.
Accordingly, when the shadow mask 90 having the protrusions or particles is aligned with the substrate 10, the protrusions or particles may contact the first electrode 47 in the pixel region P, and problems such as scratches or pits may be caused.
The shadow mask 90 is formed of a metallic material, and the shadow mask 90 for fabricating an organic electroluminescent display device, which has a large size, hangs down. Particularly, even though the shadow mask 90 is supported by the spacer 51, the shadow mask 90 hangs down in the pixel region P between adjacent spacers 51. The spacer 51 has a thickness of 2.5 micrometers to 3 micrometers. A total thickness of the buffer pattern 50 and the spacer 51, that is, a distance between a top surface of the spacer 51 and a surface of the first electrode 47 is about 6 micrometers, and the shadow mask 90 hangs down by 1 micrometer to 2 micrometers. Accordingly, when the shadow mask 90 is disposed over the substrate 10 and is aligned with the substrate 10, the scratches or pits are caused by the particles or protrusions.
To prevent the problem, the shadow mask 90 may be aligned not to contact the substrate 10, and in this case, the shadow mask 90 may be as close to the substrate 10 as possible to accurately align the shadow mask 90 and the substrate 10. A distance between the shadow mask 90 and the substrate may be 1 micrometer to 2 micrometers. However, in this case, since the shadow mask 90 is not supported by anything, the shadow mask 90 hangs down more than when the shadow mask 90 is supported by the spacer 51, and the scratches or pits are caused on the surface of the first electrode 47 in the pixel region P, where the organic emission layer 55 is formed, due to the particles or protrusions. The scratches or pits are shown as dark points in a displayed image to thereby lower display qualities.
To solve the problem, a plan of increasing the thickness of the spacer 51 has been suggested. However, the spacer 51 may be formed on a substantially entire surface of the substrate by applying a transparent organic insulating material by a spin-coating method and patterning it. To uniformly apply the organic insulating material on the substantially entire surface of the substrate, viscosity of the organic insulating material is important. If the viscosity of the organic insulating material is raised to increase the thickness of the spacer 51, spraying properties of the organic insulating material is lowered, and there is difference between thicknesses of the applied organic insulating material in central and peripheral portions of the substrate. Therefore, the spacer 51 has a maximum thickness of about 3 micrometers such that the spacer 51 has a uniform thickness all over the substrate. Additionally, when the spacer 51 has a thickness more than 3 micrometers, there are shadow problems due to the spacer 51, and step distance problems or incorrect color problems may be caused.